I debug this, but the panel and the offscreenImage had the right size. The pBuffer is big enough, too. But the offscreenImage had a black strip on the right side. So I do not really know where the problem exactly is, but this work around works here:

java-gaming.org is not responsible for the content posted by its members, including references to external websites,
and other references that may or may not have a relation with our primarily
gaming and game production oriented community.
inquiries and complaints can be sent via email to the info‑account of the
company managing the website of java‑gaming.org